Add part one of day 3

tests/Day3Test.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Tests;
+
+use AdventOfCode21\Day3;
+
+/**
+ * @internal
+ */
+class Day3Test extends AbstractTestCase
+{
+    public static int $part1ExampleResult = 198;
+    public static int $part1Result = 3309596;
+
+    public static string $part2ExampleResult = 'n/a';
+    public static string $part2Result = 'n/a';
+
+    public function setupDay(): Day3
+    {
+        return new class() extends Day3 {
+            use ReturnTestableResults;
+        };
+    }
+}